The adaptive significance of cyclomorphosis in Daphnia: more possibilities

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Abstract
SUMMARY.Morphological variability inDaphniapopulations has often been uncritically ascribed to phenotypic plasticity. For instance, detailed study revealed that the ‘cyclomorphic species’D. carinatas. l. was a complex of nine species. Several of these species often cohabit and seasonal change in their relative frequencies causes phenotypic cycles which mimic true cyclomorphosis. Intraspecific genetic variation in head shape also seems widespread and is likely to be important in explaining phenotypic changes in many single species populations.The hypothesis that helmet formation inDaphniais primarily related to predator avoidance is not supported by work on theD. carinatagroup. Seasonal trends in species composition can be explained without reference to differential predation. Natality differences exist between species with disparate head size suggesting that helmet formation may have direct effects on fitness. Two possibilities are considered. The length of the anterior adductor muscle is directly correlated with helmet size and such variation may affect swimming efficiency. In addition the laminar design of helmets suggests a role in gas exchange.

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